Screening apparatus with light reject removal

ABSTRACT

A screen for fibre pulp is provided with means for removal of light reject. The screening apparatus comprises a vertical cylindrical screen (2); and a rotor (3) which is concentrically disposed inside the screen and the substantially closed outer surface (9) of which together with the inner surface (10) of the screen defines a screening zone (11) for the pulp. A pipe (19) is provided inside the rotor (3) the upper end of the pipe being disposed adjacent to the inner surface (21) of the closed upper end (12) of the rotor (3).

TECHNICAL FIELD

The present invention relates to a screening apparatus for separatingsolid particles, such as fibre bundles used in paper making, and lightparticles from slurries, the apparatus comprising a vertical cylindricalscreen; a rotor which is concentrically disposed inside the screen andthe substantially closed outer surface of which together with theinterior surface of the screen defines a screening zone for the pulp; anaccept chamber surrounded by the outer surface of the screen; an inletfor supplying the pulp to be screened into the upper end of thescreening zone; a reject chamber connected to the lower end thescreening zone; and means for separating the heavy and the lightimpurities from each other.

BACKGROUND ART

When screening paper pulps containing a variety of impurities in ascreen described e.g. in U.S. Pat. No. 3,363,759, the reject containsalso particles which can be separated from it by employing centrifugalforce. The particles separated in this way, the specific gravity ofwhich is lower than that of the average particles, i.e. the lightreject, contains e.g. various plastics. The remaining reject containse.g. fibre bundles and pieces of knots. It is an object of the presentinvention to provide a screen which in a simple manner separates lightimpurities from the reject flow.

DISCLOSURE OF THE INVENTION

The screen to which the present invention relates has been developedfrom a known screen disclosed e.g. in U.S. Pat. No. 3,363,759. Anotherversion of the screen of the patent has later been modified for removalof plastics, this apparatus in turn being disclosed in U.S. Pat. No.3,912,622. In this version the separation of the light particles fromthe rejected pulp is arranged through a pipe one end of which is fixedto the inlet end of the screen and the other end extended through anopening in the cover of the casing. A conical extension is provided inthe other end of the pipe which extends to the open upper end of therotor. Due to the vortex movement of the rejected pulp the light rejectconcentrated in the inner part of the vortex flows out through adischarge pipe while the heavier reject gathered in the outer part isrecirculated into the screening zone. In order to achieve efficientseparation the reject flow must be ample, a fact which decreases thecapacity of the machine.

It is an object of the present invention to provide a device simple inconstruction for separating light reject from the reject flow. This isachieved by arranging a pipe inside the rotor to remove the lightreject, the upper end of the pipe being disposed adjacent to the innersurface of the closed upper end of the rotor. The invention provides asimple way of adding into existing screens a system for removing lightreject.

BRIEF DESCRIPTION OF THE DRAWINGS

The apparatus according to the invention is illustrated by theaccompanying drawings which are described in detail below.

FIG. 1 is a vertical section of an embodiment of the invention; and

FIG. 2 is a part of another embodiment of the invention.

BEST MODE OF CARRYING OUT THE INVENTION

The main parts of the screen illustrated in FIG. 1 are amulti-compartment housing 1, a screen drum 2, a rotor 3 and a shaft 4.The shaft is rotatably mounted inside the rotor on bearings on acylindrical support 5 which is fixed to the housing by a sealing joint.The rotor is mounted on the upper end of the shaft and a belt pulley 6on the lower end of the shaft for rotating the rotor. A disc 7 is fixedto the upper end of the support through which the shaft extends. Therotor outer surface 9 provided with half-spherical protrusions 8 and theinner surface 10 of the concentric screen drum defines a ring-shapedscreening zone 11 where the pulp to be screened is fed into. The upperend of the rotor is closed by an end plate 12 into which the shaft isfixed.

The upper part of the housing constitutes of a feed chamber 13 providedwith an inlet 4 for the pulp to be screened. There is a reject chamber15 provided with a discharge orifice 16 in the lower part of thehousing. The upper end of the screening zone 11 is connected to the feedchamber and the lower end to the reject chamber. There is an acceptchamber 17 outside the screen drum.

An inlet 18 for dilution water is arranged in the reject chamber. Adischarge pipe 19, which extends through the support and the disc 7 inthe upper end of the support, is disposed inside the support; the upperend of the pipe is disposed adjacent to the inner surface 21 of therotor end plate 12.

The pulp to be screened is supplied into the device through the inlet 14and at first it circulates in the feed chamber tangentially. The feedchamber acts at the same time as a scrap catcher, where stones and otherheavy pieces are removed. From the feed chamber the pulp flows into theslot 11 between the rotor 3 and the screen drum 2 which functions as ascreening zone. The accept flows from the screening zone 11 through thescreen surface 10 to the accept chamber 17. From the accept chamber thepulp is discharged through a discharge pipe 20.

The reject, i.e the portion of the pulp flow which does not pass throughthe screen surface, ends up in the reject chamber 15. In the rejectchamber the reject is diluted by dilution water supplied through theopening 18 and is discharged through the discharge opening 16.

Several mill scale test have shown that e.g. the plastics, which is atypical constituent in the light reject, remains on the reject side.E.g. by employing a 0.45 mm slot screen as the screening surface, 85 to95% of the plastics in a typical sulphate cellulose pulp can be removed,which yields accept with a plastics content of 5 to 15%, only, of theplastics content of the feed flow. The only problem that remains is howto separate the plastics and other light particles from the reject flow.

An apparatus according to the invention solves this problem. In theapparatus the light reject is gathered inside the rotor 3 in the spaceunder the top cover 12 from which it can be removed through pipe 19either continuously or intermittently.

The light reject flows into a space 22 inside the rotor through therotor bottom end 23. The high revolution speed, which is typical of therotor, brings the pulp in the screening space, in the reject chamber andin the space 22 inside the rotor into a rotating motion which createsthe centrifugal field necessary for the separation of the light reject.Dilution water may be supplied into the reject chamber in order toregulate the consistency.

FIG. 2 illustrates an alternative embodiment of the invention. The pulpis diluted at the bottom end of the screening zone 11 by liquid suppliedthrough the dilution water ring 24. There are openings 25 in the rotorthrough which the main portion of the reject flow is supplied into thespace 21 inside the rotor where the light reject is separated bycentrifugal force. The light reject flows up into the space under therotor top end 12 from which it is removed through pipe 19 and theremaining reject flows down into the reject chamber 15 from which it isdischarged through the opening 16.

A screen according to the present invention facilitates efficientseparation of light reject such as plastics although the reject flow ismaintained small in relation to the feed flow.

The invention is not limited to the embodiments presented and describedas examples, only, but several modifications may be made of it withinthe scope of protection defined by the claims.

We claim:
 1. A screening apparatus for separating the accept from thereject and separating impurities of specific gravity lower than theaverage particles of the impurities in the reject, from the heavyreject, which comprises a vertical cylindrical screen (2); a rotor (3)concentrically disposed within the screen, said rotor having an outersurface (9) and a closed upper end (12), said upper end having an innersurface (21), said closed upper end and said outer surface defining aspace (22) within the rotor, means to rotate the rotor, said screenhaving an inner surface (10), said outer surface (9) of the rotor andsaid inner surface of the screen defining a screening zone (11), anacceptor chamber (17) surrounding the outer surface of the screen; aninlet (14) for supplying the pulp to be screened into a feed chamber atthe upper end of the screening zone; a reject chamber (15) connected tothe lower end of the screening zone; and means for separating the heavyreject from the impurities of lower specific gravity from each other,which comprises a pipe (19) disposed within the rotor, the upper end ofthe pipe being located adjacent said inner surface (21) of said closedupper end (12) of the rotor, said reject collecting in the rejectchamber.
 2. A screen as recited in claim 1 wherein the pipe (19) isarranged to extend through a bearing support (5) disposed inside therotor.
 3. A screen as recited in claim 1 comprising means (24) foradmitting dilution liquid into the screening zone (11).
 4. A screen asrecited in claim 3 wherein the rotor is provided with apertures (25) foradmitting diluted reject into a space (22) inside the rotor.
 5. Thescreen according to claim 1 comprising means for admitting dilutionliquid into said reject chamber.